Conventionally, air intake in internal combustion engines such as gasoline engines has been measured by providing an air-flow sensor within an air inlet pipe in order to inject fuel in accordance with the quantity of air intake. Moreover, recently, air-flow sensors are also used for air-flow control in air conditioning equipment in a building, factory, etc., and control of wind speed and air flow etc. in environmental equipment.
There are two types of air-flow sensors: a hot type (also called a hot wire type) that utilizes change in resistance of a heated wire as a result of current flowing through a platinum element (platinum hot wire) so as to increase the temperature through self-heating, and losing heat as the air hits its heating portion; and a cold type that measures air temperature as is. Since either type of sensor needs to be placed within an air flow and to measure the temperature of the air accurately, it is necessary to control fluctuation in element resistance, fluctuation in TCR (temperature coefficient of resistance), fluctuation in how heat dissipates, and fluctuation due to turbulence of air flow around the element, etc.
Platinum is used in thin-film temperature sensors that require heat resistance as it has stable properties and excellent resistance-temperature characteristics. Such thin-film temperature sensors have lead wires welded to electrode parts, and an insulating material, such as glass or a conductive material including a precious metal for reinforcing connections applied on the welded parts so as to strengthen the connections. Patent Document 1, for example, discloses a thin-film thermistor having an Au—Pt layer formed on welded parts of an Au—Pt electrode film and lead wires on a substrate, and further covered with a glass layer so as to improve conductivity and heat resistance.
Patent Document 2 discloses a technique of providing electrode pads made of an Au—Pt alloy on the substrate of a gas sensor, connecting the lead wires to the pads for the purpose of improving durability of the gas sensor against change in temperature, maintaining connection strength of lead wires to the electrode pads, and further increasing the connection strength of the lead wires, and then covering them with a protective film.